The U.S. National Security Agency, which gathered the information by monitoring international phone calls and e-mails of some Americans, provided the tips, including names, phone numbers and e-mail addresses. It was then the FBI's job to investigate them.

While the administration said the program was effective, the Times reported that current and former anti-terrorism investigators said few of the tips were useful, and agents were forced to stop other work to follow them.

"We'd chase a number, find it's a school teacher with no indication they've ever been involved in international terrorism – case closed," a former unidentified FBI official said.

However, "I can say unequivocally that we have gotten information through this program that would not otherwise have been available," the Times quoted NSA director Gen. Michael Hayden as saying.

The program may have turned up people with links to al-Qaeda that the FBI had not previously known about, and may have led to arrests outside the U.S.

The FBI complained to the NSA about the volume of tips.

The agency also raised issues about the legality of spying on Americans, which is illegal inside the U.S. without the approval of a special secret court.

But the Justice Department told the FBI that it was legal, the paper said.